Electron stream focusing



Sept. 14, 1965 R. M. PHILLIPS 3,206,635

ELECTRON STREAM FOCUSINCT Filed April 27, 1961 2 Sheets-Sheet 1 )POBEET M 2% A A 5 INVEN TOR.

BYM' /V M ATTORNEY FIE- Sept. 14, 1965 M. PHILLIPS ELECTRON STREAM FOCUSING Filed April 27. 1961 2 Sheets-Sheet 2 Payer M PM; 5

IN V EN TOR.

BY 2%? w ATTORNEY United States Patent 3,206,635 ELECTRON STREAM FOCUSING Robert M. Phillips, Redwood City, Calif., assignor to General Electric Company, a corporation of New York Filed Apr. 27, 1961, Ser. No. 106,006 6 Claims. (Cl. 31539) This invention relates to systems for focusing hollow streams of charged particles, and more particularly to such systems in which a hollow electron stream having the configuration of a hollow right circular cylinder is collimated by magnetic fields for travel over a relatively long path. Such electron streams are used in some electron tubes; e.g., in some forms of traveling-wave tubes. The focusing system is particularly well suited to focusing such a stream of electrons and simultaneously imparting a sideto-side undulatory motion to the electrons in the stream.

It is an object of the invention to provide an improved system for focusing an electron stream which has the configuration of a hollow right circular cylinder.

In a traveling-wave tube, an electron stream is made to interact with a traveling electromagnetic wave over a distance which is a plurality of operating wavelengths long. In order to accomplish this an electron stream is projected down an interaction region in which the electromagnetic wave is propagating. In the co-pending application, S.N. 816,540, filed May 28, 1959, now patent No. 3,129,356, in the name of the present inventor and assigned to the assignee of the present invention, a traveling-wave tube is disclosed wherein the electromagnetic waves are propagated in an interaction region and an electron stream is projected along the length of the region in such a manner that electrons in the stream have a transverse velocity the direction of which varies periodically along the guide. A radio frequency electric field is produced in the waveguide with a phase velocity related to the stream velocity and of a mode such that a transverse com ponent of the radio frequency electric field produces a modulation in the transverse velocity of electrons in the stream. This modulation is converted into modulation in the axial velocity of the electrons by a non-time varying focusing force such as a non-time varying spatially periodic magnetic field. Such a field converts changes in the transverse momentum of the electrons in the stream into changes in the axial momentum while leaving the stream energy essentially unchanged. The momentum conversion thus changes transverse velocity modulation in the stream into axial electron bunching, that is, bunching of electrons along the axial length of the stream. The transverse component of electric field in the radio frequency wave abstracts energy from the transverse momentum of the beam. Thus, the ultimate source of energy causing the radio frequency Wave to grow is the axial velocity of the stream. Stated in another way, it may be said that the interaction mechanism depends upon an intermediate momentum conversion, that is, conversion of the axial momentum of the stream into transverse momentum with the subsequent interchange in energy between the transverse components of the electric field in the radio frequency wave and transverse momentum of electrons in the stream.

One of the embodiments of the traveling-wave tube disclosed in the co-pending application discussed above provides for interaction in a coaxial waveguide wherein the interaction region has the general configuration of a right circular cylinder. The requirements for the interaction are met if electromagnetic waves are propagated in the region in a coaxial waveguide mode of the transverse electric 01 (TE type and an electron stream which also has a configuration of a hollow right circular cylinder is projected down the length of the interaction region with the electrons in the stream undulating from side-to-side so as to exhibit substatnially equal velocity swings in both directions. The motion of the individual electron may be visualized by considering an electron in the interaction region at a given distance from the center of the right circular cylinder undulating from side-to-side without changing its distance from the center of the cylinder as it moves down the length of the interaction region. In other words, the individual electrons in the stream have the oscillatory side-to-side motion of a pendulum while at the same time moving down the length of the interaction region.

It is just such an electron stream that the focusing system contemplated here is intended to focus. That is, it is an object of the invention to provide an improved charged particle collimating system for focusing an electron stream which has the configuration of a hollow right circular cylinder and simultaneously imparting a side-toside undulatory motion to the electrons in the stream.

One way to provide a time-constant spatially alternating longitudinal magnetic field along the length of an interaction region is to use a series of magnets around the outside of the tube and spaced along the length of the tube to provide the desired period of spacing and making alternate magnets of different polarity. This approach to focusing is commonly known as periodic magnetic focusing. If the magnets are permanent magnets this approach is called periodic permanent magnet focusing. The name, of course, comes from the fact that the focusing magnetic field produced along the electron stream path varies spatially in a periodic fashion. This type of focusing is described in an article by J. T. Mendel, C. F. Quate, and W. H. Yokum, Electron Beam Focusing With Periodic Magnetic Fields, which appears in the Proceedings of the IRE, May 1954, pp. 800 through 810, inclusive. In practice it is possible to provide the non-time varying spatially periodic magnetic focusing fields with solenoids but the use of periodic permanent magnets has so many advantages from the standpoint of weight, size, and power consumption that it is much more practical for most applications to use permanent magnets.

Since the use of magnetic fields and particularly nontime varying spatially periodic magnetic focusing fields for this purpose of focusing electron streams is so well known at the present time the detailed analysis of the forces required are not made here. However, it is necessary to have a few of the basic concepts in mind in order to understand the present invention. For example one must recognize that electrons moving along magnetic lines of force are not displaced but if they possess a component of velocity perpendicular to the magnetic lines of force, they experience a force which is perpendicular to both the velocity component in question, and to the magnetic lines of force. Thus, if the magnetomotive force provides lines of magnetic force in the region traversed by the electron stream wherein the lines of force are substantially parallel to the direction of desired electron flow the electrons will tend to move along the lines of magnetic force as long as the magnetic field is strong enough to overcome space charge forces between the electrons in the stream. Under these conditions the electrons tend to remain focused and move in the desired direction as a stream.

Any periodic permanent magnet focusing system has lines of magnetic force directly under the pole pieces which must necessarily be essentially in a radial direction instead of in a longitudinal direction. Since the charged particles (e.g. the electrons in the electron stream) which possess a component of velocity perpendicular to the magnetic lines of force experience a force perpendicular to both the velocity component and to the magnetic lines of force the particles tend to curl around the line (either in a clockwise or counter clockwise direction depending upon the direction of the lines of magnetic force) but the force due to the radial magnetic field does not tend to de-focus the electrons, but causes them to undulate alternately in clockwise and counter clockwise directions as they move under the pole pieces of the system which are of opposite polarity. Thus, the periodic magnetic focusing arrangement is very useful for the fast wave interaction described in the Phillips co-pending application supra.

The type of interaction in question has some very particular and exacting requirements. For example, for best interaction the hollow stream used should fill up to one-third of the volume between inner and outer conductors, the stream should lie approximately half way between the inner and outer conductors, electrons in the stream should exhibit equal velocity swings in undulating in the clockwise and counter clockwise direction, the percent of the total electron stream energy in spin should be a reasonably large fraction of the total energy (e.g., 30 percent) and all electrons at a particular cross section of the beam should experience approximately the same velocity swing and spin energy. The last two requirements in particular call for a large radial magnetic field as well as a sufficient longitudinal field to focus the stream. In addition to these requirements, there is a limitation on the amount of radial variation which can be tolerated in the radial magnetic field. That is to say, the radial magnetic field should not vary greatly along the cross section of the interaction region.

Other electron stream periodic magnetic focusing systems will fulfill some of these requirements. However, most of these systems provide radial magnetic fields which are not very strong and which vary radically.

The radial magnetic fields of such circuits may be bolstered by the addition of a magnetic circuit in the center conductor of the coaxial arrangement. However, this arrangement prevents adequate cooling of the center conductor in high power application and is cumbersome. Therefore, introduction of a magnetomotive force along the center of the coaxial interaction region is not practical for most applications.

Accordingly, it is an object of the invention to provide a circuit for collimating a hollow electron stream which produces non-time varying spatially periodic longitudinal magnetic fields and relatively strong longitudinally spaced radial alternating magnetic fields which do not vary appreciably radially.

Another object of the invention is to provide such a circuit which does not require the addition of magnets inside the hollow electron stream.

In carrying out the present invention, an electron stream having the configuration of a hollow right circular cylinder is focused and the electrons therein are simultaneously given a side-to-side undulatory motion by providing a time constant spatially alternating longitudinal magnetic field along the path of electron flow which varies in intensity radially and relatively strong radial magnetic fields spaced at intervals down the length of the device which radial fields do not vary appreciably in the radial direction.

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIGURE 1 shows somewhat schematically a cross sectional view of a traveling-Wave tube which incorporates the electron stream focusing system of the present invention;

FIGURE 2 is an enlarged view of the portion of the system within the section lines 2-2 of FIGURE 1;

FIGURE 3 is a transver e e ti n through the body of 4 the system taken along section lines 33 of FIGURE 2 (to the same scale as FIGURE 2);

FIGURES 4 and 6 respectively are central vertical lon gitudinal sections through segments of dilferent embodiments of electron stream collimating systems; and

FIGURES 5 and 7 are transverse sections through the systems illustrated in FIGURES 4 and 6 respectively.

Referring specifically to FIGURE 1 of the drawings an electron tube 10 is illustrated which is suitable for providing interaction of the fast wave type referred to in in the Phillips application previously referenced. At opposite ends of the evacuated envelope 11 are positioned an electron gun 12 and a target or collector electrode 13. Between the electron gun 12 and the collector 13 is a long tubular main body portion 14 of circular crosssection wherein the interaction takes place between the electron stream and the electromagnetic waves. The vacuum tight envelope 11 is of a metal ceramic construction and is composed principally of the collector 13, main body portion 14 and the electron gun enclosure 15.

The tubular main body portion 14 of the tube 10 is formed of a central metallic tube 15 is joined at opposite ends to annular ceramic input and output windows 16 and 17 respectively. The output window 17 is provided at the output or collector end of the electron tube and the input window is positioned at the input or gun end of the tube. These windows are provided in the envelope 11 for the introduction and extraction of electromagnetic waves.

The input window 16 is sealed to the gun enclosure 15 by a vacuum tight seal. The electron gun enclosure includes a bulb like tubular metallic member 18 which has a small cylindrical tubular portion at one end that is of the proper size to form the metal to ceramic seal with the input window 16, a conical portion which extends outwardly from the small tubular portion and ends in a larger cylindrical tubular portion that is of a proper size to surround the electron gun structure. As illustrated the open end of the large cylindrical portion is closed by a ceramic disc 19 which fits inside the enlarged tubular section and is sealed thereto in a vacuum tight fashion. As will be discussed more fully later, connections to internal tube elements are brought out through the end closing disc 19 but appropriate vacuum seals are provided in each case so that the tube is vacuum tight.

As is illustrated the output window 17 is sealed to the collector electrode 13 by means of a metallic tubular sec tion 20. Again the seals are vacuum tight. The collector 13 is provided with an aperture 21 in the end opposite the window which aperture provides access to an internal tube element as described subsequently. The aperture 21 is closed in such a manner that it will hold a vacuum. It is to be noted that the metal parts of the vacuum tube are generally stainless steel, copper or other non-magnetic material. The collector 13 is made of copper or other such metal which has good heat conducting properties so that it dissipates the heat generated by electron bombardment. The heat dissipating properties of the collector 13 are enhanced by providing protruding annular fins 22 around the outer surface.

The electron gun 12 comprises an annular electron emissive cathode 23, a heater unit for the cathode (not shown) and an electrode system 24 for shaping and accelerating the electron stream. Conductors 25 are brought out from the electrode elements of the electron gun 12 in order to apply the accelerating and stream shaping voltages to the electrodes of the gun 12. As illustrated, the electrons emitted from the cathode are formed into a convergent conical annular stream. The lead in conductors and gun elements are shown schematically and are not described in detail since the gun itself does not form a part of the present invention. Any convergent non-immersed flow electron gun for forming a hollow stream of electrons may !be used.

The propagating circuit for the tube 10 is a coaxial waveguide. The outer conductor of the circuit comprises the tubular metallic center portion 15 of the envelope and the center conductor is a conductive rod 26 which extends down the full length of the tube. The conductive center conductor 26 is supported in its coaxial position at its op- The collector end of the center conductor on the outside of the outer conductor 15 produce a magnetic field having lines of force which extend between each of the magnets 30 and the center conductive rod 26. By convention we assume that the lines of force go from north to south, hence the first magnet encountered (a po'site ends. 5 26 is supported in an annular supporting an insulating magnetic north pole) produces lines of force which enter ceramic ring 27 which is sealed in the centrally located the interaction region and go from the magnet to the cenaperture 21 in the collector 13. All seals around the ceter conductor 26. The center magnet, a magnetic south ra'mic ring 27 are made vacuum tight. The opposite end pole has lines of force which enter the magnet from the of the conductive rod 26 is held by another annular sup- 10 center conductor 26 and the third magnet shown (again porting and insulating ceramic ring 28 which is held in a magnetic north pole) has lines of force which go from position within a centrally located cylindrical lead out for the magnet to the center conductor 26. the center one of the gun electrodes which in turn is We may first consider the focusing action on a more or sealed in position in a centrally located aperture in the less intuitive basis in order to get a feel for the action of ceramic end disc 19 of the gun enclosure 25. The inner 15 the circuit. First, it will be recognized that the radial and outer surfaces of inner and outer conductors 26 and magnetic field (B is a maximum directly under each of 15 respectively define the interaction region which has the magnetic poles and varies only slightly with radius the configuration of aright circular cylinder. due to the use of the highly permeable material in the In order to collimate the stream of electrons trom the center of the tube. Hence, all electrons experience apgun 12, a magnetic focusing system is provided which in- 20 proximately the same spin and centrifugal force in passcludes a means to provide a time-constant spatially aling under a pole. The strength of the radial magnetic ternating longitudinal magnetic field in the interaction field is essentially the same under adjacent magnetic poles region nd a mean to support the magnetic field and in but opposite in direction. Hence, the electrons passing effect shape the magnetic field in the interaction region. under each successive pole experience approximately the The time constant spatially alternating magnetic field is same spin or velocity swing in opposite directions. Thus, produced in the embodiment of FIGURES 1, 2 nd 3 by electrons experience an undulating or pendulum like moa serie of radially magnetized annular permanent 'magtion as they move down the length of the interaction nets which are positioned around the outer conductor region. 15 and spaced apart along its length. The magnets 30 The longitudinal magnetic field is a maximum (B are arranged so that the polarity of adjacent magnets are 30 half y between the magnets 30 and the Variation of the o p ite, In th illu tration the polarity of h f th longitudinal magnetic field with radius is considerable. magnets along its inner periphery is indicated by the let- For example, the longitudinal magnetic field is essentially ters N and S which appear at the outer periphery of the zero at the inner conductor (r and reaches a maximum magnet Thi i d as a att f convenience. The at the inner surface of the outer conductor (at r Thus spacing between magnets 30 is determined :by the interacelectrons traveling down the length Of the interaction tion requirements. For example, in the fast wave interg n are SPun s de-to-side in an un'dulatory motion and action the electrons in the electron stream must undulate between the pole pieces they cross a longitudinal line of from side-to-side in such a manner that the velocity dimagnetic force which tend to curl or force the electron rection of electrons in the stream vary periodically at a in toward the center of the device. At the same time an rate such that an individual electron in the stream travels 40 electron on the outer periphery of the cylindrical electron one periodic length P in the time that the wave travels stream (at r experiences an outward force due to space one periodic length P plus one w v length f the l etr charge and an outward force due to centrifugal force as magnetic wave. Thus, the field producing magnets 30 it undulates or spins around the center of the device. The are spaced apart so that the distance between the center electrons on the inner periphery of the hollow stream (at of every second magnet is the periodic length P (see FIG- r r experience an inward space charge force and an out- URES 1 and 2 of the drawings). 40 ward centrifugal force. Thus, in order to 'be able to focus The magnetic field supporting and shaping means is, in the hollow stream in this structure it is necessary to arthe tube illustrated, the center conductor 26 of the coaxial ge t e dimensions such that the inward focusing force waveguide. This conductor 26 is made of a material of the longitudinal magnetic field for the outer most elecwhich has a high magnetic permeability such as cold rolled r trons just balances the outward centrifugal and space steel. The center conductor 26 or magnetically permecharge forces; and the outward centrifugal force just balahle field h p and focusing rod acts as a magnetic ances the inward magnetic (from longitudinal magnetic eqlll-potehtlel Surfaeeof Course, neel not he Solid; 3 field) and space charge forces for the inner most electron. Shell of high Permeability material 'Will Sulllee as long as As it turns out these dimensions can be tabulated rather saturation does not become appreciable. Further, the welL condiliactivity of thebouter surface of the conguctrve rod 26 For those who wish to follow the dasign procedure a fizz gfi i i gpig f g i g gfzgg ggg more mathematical treatment is given below. The probnetic focusing circuit depends in large measure upon the 1s ilind the k.mgltudlnal magnet: field strength (B2) variation of the radial magnetic field with radius (only an R 1a magnet: field stfength Structure a slight variation) and the longitudinal magnetic field These may be found Y Solvmg Laplace S equajlhlon for the with radius (which is appreciable). In Order to unvdep scalar magnetic potential for boundary conditions at the Stand this f t the fi ld configurations are fi t com surface of the inner conductor (r,) and the inner surface sidered in a general, non-rigorous way by use of the scheof the Outer Conductor ("0) and finding the magnetizing matie diagram of FIGURE 2, H r i it seen th t th force or field intensity (H) from the gradient of the scalar radially magnetized sources of magnetomotive force 30 potential. The results are:

where E is the magnetic field strength (longitudinal) between magnetic pole pieces and at the inner circumference;

sis the distance between adjacent magnets (see FIGURES 1 and 2 of the drawings);

P is the magnetic period, i.c., the distance between centers of every second magnetic pole piece (see FIGURES 1 and 2 of the drawings);

r is the radius being considered (in these equations may be from the outer surface of the inner conductor,

r;, to the inner surface of the outer conductor r and the I and K expressions represent modified Bessel functions as defined in Morse and Feshbach, Methods of Theoretical Physics, McGraw-Hill, 1953, page 1323.

In order to obtain an idea of the required dimensions for beam focusing, a balance of forces at a transverse plane half way between pole pieces were obtained using the general conditions of balance described above and making some additional assumptions. For example, it was initially assumed that the ratio of the radii of the inner and outer conductors (r r is near enough to unity so that the inward space charge force acting at the inner periphery of the beam (at r equals the outward space charge force at the outer periphery of the beam (at r The balance of forces described above requires that F 02 s+ m2 where F is a magnetic focusing force; F is a space charge force; and F is a centrifugal force.

The numerals in the subscript define the point where the particular type of force is considered. For example, F is the centrifugal force and P is the centrifugal force at the inner boundary of the electron stream (at r Solving these two equations simultaneously gives c1+ c2 m1+ 2 The centrifugal forces are given by F :mrti where m is the mass of the particles considered;

r is the radius at which the centrifugal force is being considered; and

0 is the angular velocity of the electrons in radians per second.

The magnetic focusing force is given by where m, r, 0, and B are the same as the symbols considered above and 1; is the charge to mass ratio e/m The particular typical geometry was assumed where the ratio of the distance between poles (s) to the period (P) 1s and the ratio of the radii of the inner conductor to the outer conductor is A trial and error solution to the above equations gives a required force balance. It was assumed that the beam thickness was /3 the diiference between the inner diameter of the outer conductor and the outer diameter of the inner conductor that is The resulting value of the radius of the inner conductor to the period r /P turns out to be A rough design calculation to determine the field required to focus a significant beam in an S-band coaxial fast wave structure produced the following results:

Assumed values r =3 inches, r =6 inches. Beam thickness 1 inch.

Beam perveance 10X 10".

Peak B B =B =400 gauss.

Peak spin velocity V /c=0.45.

Stopband voltage 40 kv., approximately.

In some instances it may prove to be advantageous to operate with dimensions other than those dictated by beam focusing requirements. Two straight forward modifications of the previously considered structure are illustrated in FIGURES 4 and 6. In each case the same type focusing magnets are utilized with an interaction region which has the shape of a hollow right circular cylinder and the center conductor 26 of the structure is made of a high permeability material. However, in the embodiment of FIGURES 4 and 5 the pole pieces are brought through the outer wall 15 of the tube with the result that the beam is in closer proximity to the pole pieces than the outer conductor 15.

In the embodiment of FIGURES 6 and 7 the pole pieces (of magnets 30) extend inward and the center conductor 26 extends outward in a symmetrical fashion under the pole pieces. This embodiment makes a choice of the radium of the inner conductor to the edge even more flexible.

One may wish to add an electric field between the inner and outer conductor. This can be readily done since the center conductor 26 is insulated from the other tube parts. This case, i.c., including the electric field, is one which was proposed in the co-pending Phillips application supra.

Thus it is seen that the proposed period hollow beam focusing circuit focuses the electron stream without the necessity of having an electric field between inner and outer conductors or without the provision of a periodic magnetic circuit within the inner conductor. The properties of the electron stream focused by the circuit (symmetrical spin angular velocity, etc.) are those required by the undulating beam fast wave interaction.

While particular embodiments of the invention have been shown it will, of course, be understood that the invention is not limited thereto since many modifications, both in the circuit arrangement and the instrumentalities employed may be made. It is contemplated that the appended claims will cover any such modifications as fall within the true spirit and scope of the invention.

What I claim is new and desire to secure by Letters Patent of the United States is:

1. A device for effecting energy interchange between high frequency electromagnetic waves and an electron stream comprising:

an elongated circularly apertured magnetomotive force generating member forming a succession of axial and radial magnetic fields alternating along the length of the circular aperture thereof;

a continuous cylindrical rod of high magnetic permeability concentrically disposed within the circular aperture of said member;

means including electron gun and collector means for projecting an electron stream between the inner surface of said member and the outer surface of said rod;

and means for coupling electromagnetic energy into said device including means for launching an electromagnetic wave to travel along the length of said 9 device between the inner surface of said member and the outer surface of said rod, said wave having an electric field component oriented transversely to the longitudinal dimension of the device.

2. The device of claim 10 wherein said electron stream progresses one magnetic period along the length of said device in the time that the wave travels one magnetic period plus one wave length of the wave.

3. The device of claim 2 wherein said means for launching is disposed near one end of said member.

4. The device of claim 3 wherein an output transmission means is disposed near the other end of said member.

5. A device for effecting energy interchange between high frequency electromagnetic waves and an electron stream comprising:

an elongated circularly apertured magnetomotive force generating member forming a succession of axial and radial magnetic fields alternating along the length of the circular aperture thereof;

a continuous cylindrical rod of high magnetic permeability concentrically disposed within the circular aperture of said member,

means including electron gun and collector means for projecting an electron stream along the length of said device between the inner surface of said member and the outer surface of said rod;

and means for coupling electromagnetic energy into said device including means for launching an electromagnetic wave to travel in a coaxial waveguide mode with transverse electric field along the length of said device between the inner surface of said member and the outer surface of said rod.

6. The device of claim 5 wherein said electron stream 10 progresses one magnetic period along the length of said device in the time that the wave travels one magnetic period plus one wave length of the wave.

References Cited by the Examiner UNITED STATES PATENTS HERMAN KARL SAALBACH, Primary Examiner.

ARTHUR GAUSS, Examiner. 

1. A DEVICE FOR EFFECTING ENERGY INTERCHANGE BETWEEN HIGH FREQUENCY ELECTROMAGNETIC WAVES AND AN ELECTRON STREAM COMPRISING: AN ELONGATED CIRCULARLY APERTURED MAGNETOMOTIVE FORCE GENERATING MEMBER FORMING A SUCCESSION OF AXIAL AND RADIAL MAGNETIC FIELDS ALTERNATING ALONG THE LENGTH OF THE CIRCULAR APERTURE THEREOF; A CONTINUOUS CYLINDRICALLY ROD OF HIGH MAGNETIC PERMEABILITY CONCENTRICALLY DISPOSED WITHIN THE CIRCULAR APERTURE OF SAID MEMBER; MEANS INCLUDING ELECTRON GUN AND COLLECTOR MEANS FOR PROJECTING AN ELECTRON STREAM BETWEEN THE INNER SURFACE OF SAID MEMBER AND THE OUTER SURFACE OF SAID ROD; AND MEANS FOR COUPLING ELECTROMAGNETIC ENERGY INTO SAID DEVICE INCLUDING MEANS FOR LAUNCHING AN ELECTROMAGNETIC WAVE TO TRAVEL ALONG THE LENGTH OF SAID DEVICE BETWEEN THE INNER SURFACE OF SAID MEMBER AND THE OUTER SURFACE OF SAID ROD, SAID WAVE HAVING AN ELECTRIC FIELD COMPONENT ORIENTED TRANSVERSELY TOTHE LONGITUDINAL DIMENSION OF THE DEVICE. 